Field of the Invention
The present invention relates to a field device.
Priority is claimed on Japanese Patent Application No. 2015-089313, filed Apr. 24, 2015, the contents of which are incorporated herein by reference.
Description of Related Art
In a plant or a factory or the like, a distributed control system (DCS) has been implemented to realize a high level of automated operation. In the DCS, on-site devices, which are referred to as field devices, are connected via a communication means to controllers, which control the field devices. The field devices used in the DCS are classified into measures such as flowmeters and temperature sensors, and operating devices such as valve devices and actuator devices.
The measure that is a type of the field device generally includes sensors that detect a variety of state quantities (e.g., flow rate, temperature, etc.), an amplifier that amplifies detection signals of the sensors, an analog-digital (A/D) converter that converts the amplified detection signals to digital signals, a calculator that converts the digital signals to process values, and an output that outputs the process values to the outside. Also, the operating device that is a type of the field device basically performs an operation based on the control of the control device, and may include sensors and an A/D converter like the measure. For example, the valve device may include sensors and an A/D converter in order to detect the actual degree of opening of the valve.
Patent Literature 1 (Japanese Unexamined Patent Application, First Publication No. 2012-93168), Patent Literature 2 (Japanese Unexamined Patent Application, First Publication No. 2011-33491), and Patent Literature 3 (Japanese Unexamined Patent Application, First Publication No. 2010-276470) disclose an example of a conventional electromagnetic flowmeter that is a type of a field device. The electromagnetic flowmeter, for example, detects an electromotive force (an electromotive force proportional to a flow velocity) generated by applying a magnetic field to a fluid flowing in a pipe, and measures the flow rate of the fluid. The electromagnetic flowmeter includes components (components equivalent to the sensors, the amplifier, the A/D converter, the calculator, and the output) that converts a result of detecting the electromotive force to a process value (a flow rate) and outputs the converted result to the outside.
A representative of an A/D converter provided in a field device includes a successive comparison type A/D converter and a ΔΣ type A/D converter. In all these A/D converters, there occurs a delay (a group delay) corresponding to a time required for a conversion process of converting analog signals to digital signals. For this reason, for example, when a calculator provided in the field device acquires in the digital signals converted by the A/D converter, it is thought that it is necessary to perform the acquisition in consideration of the group delay.
The successive comparison type A/D converter makes it possible to control a section (hereinafter referred to as “sampling section”) in which the analog signals are converted into the digital signals, and only the digital signals converted within the sampling section is output from the successive comparison type A/D converter. For this reason, when the successive comparison type A/D converter acquires the digital signals converted within the sampling section, there is no need to consider the group delay. However, the ΔΣ type A/D converter cannot control the sampling section as in the successive comparison type A/D converter, and the digital signals are continuously output from the ΔΣ type A/D converter. For this reason, there is a need to strictly consider the group delay in order to acquire only digital signals required for accurate calculation among the digital signals continuously output from the ΔΣ type A/D converter.
For this reason, there are some cases where, in a field device including an A/D converter (the A/D converter at which the group delay occurs, which cannot control the sampling section and continues to perform the conversion process) like the ΔΣ type A/D converter, it is difficult to accurately acquire only digital signals converted in synchronization with certain cyclic signals. For example, in the electromagnetic flowmeters disclosed in Patent Literatures 1 to 3 above, to avoid an influence of differential noises, only a specific section of the detection signals acquired from the sensor in synchronization with excitation signals (signals for applying a magnetic field to a fluid) is converted to the digital signals. However, there are some cases where, if an A/D converter provided in the electromagnetic flowmeter is, for example, the ΔΣ type A/D converter, it is difficult to accurately acquire only digital signals within the specific section for the aforementioned reason.